It is often necessary to quickly obtain a sample of blood and perform an analysis of the blood sample. One example in which there is a need for obtaining a sample of blood is in connection with a blood glucose monitoring system where a user must frequently use the system to monitor the user's blood glucose level.
Those who have irregular blood glucose concentration levels are medically required to regularly self-monitor their blood glucose concentration level. An irregular blood glucose level can be brought on by a variety of reasons including illness such as diabetes. The purpose of monitoring the blood glucose concentration level is to determine the blood glucose concentration level and then to take corrective action, based upon whether the level is too high or too low, to bring the level back within a normal range. The failure to take corrective action can have serious implications. When blood glucose levels drop too low—a condition known as hypoglycemia—a person can become nervous, shaky and confused. That person's judgment may become impaired and that person may eventually pass out. A person can also become very ill if his blood glucose level becomes too high—a condition known as hyperglycemia. Both conditions, hypoglycemia and hyperglycemia, are potentially life-threatening emergencies. Therefore obtaining accurate test results is highly important.
One method of monitoring a person's blood glucose level is by portable hand-held blood glucose testing devices. The portable nature of these devices enables the user to conveniently test his blood glucose level wherever the user may be. To check the blood glucose level a drop of blood is obtained from him, for example from the fingertip, using a separate lancing device. Once the requisite amount of blood is produced on the fingertip, the blood is harvested using the blood glucose-testing device. The blood is drawn inside the testing device, which then determined the concentration of glucose in the blood. The results of the test are communicated to the user by a display on the testing device. More detail concerning lancing devices is set forth in U.S. Pat. No. 6,152,942, which is commonly assigned and incorporated herein by reference in its entirety.
Drawbacks associated with optical instruments for reading calorimetric reactions include size, low signal throughput and accuracy errors which are due, in part, to mechanical alignment (or mis-alignment) sensitivity of the optical components. These problems are further compounded when the optical instruments require readings at more than one wavelength or at multiple wavelengths. Providing multiple wavelengths compound these problems because prior art devices produce light of each wavelength with a different light element such as a light emitting diode. Each of the light emitting diodes can not be linearly aligned, or identically aligned, with the sample. This results in the light from each of the light emitting diodes having a different intensity and different intensity distribution across the sample.
Many glucose-monitoring systems determine a concentration of glucose in the blood sample by measuring the diffused reflectance from a reagent. The reagent has a color change that is proportional to the concentration of glucose in the blood sample. Generally, diffused reflectance is the preferred method of reading the change in color of the reagent. Additional background concerning colorimetric testing and diffuse light reflectance is found in U.S. Pat. Nos. 5,723,284; 6,181,417B1; 5,518,689; 5,611,999, all of which are incorporated herein by reference in their entirety.
Current methods of reading diffuse reflectance use LEDs as a monochromatic source of illumination. The problem with using an LED is that a typical center wavelength tolerance of plus or minus 20 nm causes a variation in the diffused reflectance. The variation in wavelength around the center wavelength will cause the reagent color to vary around a reflectance corresponding to the center wavelength of the LED. This reflectance variation translates into an error in glucose concentration. An error in glucose concentration level can lead the user to take too much medicine or avoid taking enough medicine, thereby resulting in a potential seizure, coma, or even death. Thus obtaining accurate glucose concentration levels in a blood sample is critical.
One category of diffuse reflectance is two-wavelength diffuse reflectance. Current designs of two-wavelength diffuse reflectance readheads use coaxial sample illumination from LEDs at two different wavelengths. The coaxial illumination of the sample by the two LEDs is traditionally done with a beam splitter. Another method is to illuminate the sample with both LEDs tilted 15 degrees off the sample's normal optical axis.
One method for reducing the reflectance variation due to tolerance of the LED is to sort the LEDs according to tight center wavelength tolerances to reduce the spectral errors. Such sorting processes can increase the cost of LEDs by up to 15 times their nominal cost. A low cost alternative to reducing the spectral errors caused by LED center wavelength variation is taught herein. A method of coaxially illuminating the sample to be analyzed is also taught herein. Additional advantages concerning illumination, detection and blood monitoring, generally, will be apparent to those of ordinary skill in the art from the teachings herein.